A small-cell base station is one of the hot issues regarding the fifth-generation (5G) wireless system in the network communication industry. Due to the densely distributed requirement with the use of small-cell base stations, the small-cell base stations need to intensively coordinate with one another to avoid mutual interference of signals to and from respective antennas. The intense coordination adversely affects the performance of the network communication. Therefore, in the 5G Ultra-Dense Network (UDN) environment, the cross-station coordination is generally conducted by way of a network multi-input multi-output (network MIMO) system. Accordingly, the interference and deterioration of communication caused by analogous servo antennas as a result of the increase in number of small-cell base stations could be alleviated.
In order to solve the above-mentioned problems, the prior art uses a precoding technology for communication. With the precoding technology, an inverse matrix is obtained. A signal, before being transmitted from a base station via a servo antenna, is previously processed with the inverse matrix. By way of such a previous processing operation, the phases and amplitudes of the signals to be transmitted via multiple antennas are adjusted so that an antenna of a user device, e.g. a mobile phone, can receive the signals as a constructive synthetic wave with an improved intensity.
It is understood that since the precoding operation is a reverse-matrix operation, the operational complexity would be O(n3), where n is the number of emitting antennas. Taking a 20 MHz time-division duplex (TDD) mode as an example of an uplink-downlink configuration 2 of the 3GPP, the calculation would be executed once per 15 KHz band (i.e. per subcarrier), and up to 720000 precoding operations would be executed per second. Therefore, without improving the precoding technology, there would be problems in increasing the number of collaborative base stations in a Network MIMO system and it would hard to establish a virtual super base station to serve more and more users.